This has been discussed before and even if your company requires you to get a "doctors note" all you need is something saying you went to the doctor. They do not have the legal right to know more than you just went to the doctor.
I am not sure if it is true or not but I heard of a guy calling in sick 12 times in one year and he was told he would be fired if he did it again. He called in sick again and nothing happened. Has anyone ever heard of someone actually being fired for sick calls?